Here are some tips on how to speed up the agenda generation. For those
tips that depend on a specific version of Org, we mention this version.

Reduce the number of Org agenda files

The more agenda files, the more time it takes to check all of them before
producing an agenda command. The older your hardrive is, the longer it
takes to visit a file. Don't buy a new harddrive now! Just reduce the
number of agenda files.

Also, don't forget that you can define the set of agenda files for each
agenda custom command like this:

Reduce the number of DONE and archived headlines

When matching against TODO-type tasks, Org will skip the ones marked as
DONE or archived. If you have many DONE tasks and archived tasks in your
file, better to store them in another file.

Inhibit the dimming of blocked tasks

By default org-agenda-dim-blocked-tasks is set to t, which will dim blocked
tasks. For the agenda to get the relevant information, it needs to check
against the headline up, and this takes time. If you don't need this
feature globally or for a specific agenda, turning it off will speed up
agenda generation.

Inhibit agenda files startup options (Org > 8.0)

When you run an agenda command, Org visits agenda files that are not yet
visited. When finding a file for the first time, Org checks the startup
options and apply them to the buffer: those options are either globally set
through the org-startup-* variables or on a per-file basis through the
#+STARTUP keyword.

Especially, Org will honor the startup visibility status, as set by
org-startup-folded or #+STARTUP: folded.

This may slow down the operation of visiting a file very much, and the
process of selecting agenda entries consequently.

To prevent agenda commands to honor startup options when visiting an agenda
file for the first time, use this:

(setq org-agenda-inhibit-startup t)

The side-effect is that newly visited file will have all their headlines
visible, but this speeds up agenda generation a lot when those files have
many nested headlines.

Disable tag inheritance in agendas (Org > 8.0)

Defining inherited tags for a headline in the agenda takes time, because
Org needs will grab inherited tags from higher level headlines.

org-use-tag-inheritance controls whether tags are inherited for tags-type
agenda commands: tags, tags-todo and tags-tree. This variable also
controls whether tags are inherited when running the command M-x
org-sparse-tree RET in an Org buffer (hence the name of this variable,
without the org-agenda- prefix.)

Generally, you want this variable to be nil, because the headlines on
those agenda types don't depend on tags (and a fortiori on inherited
ones.) Still, the default is to use tags in all agenda types, because
org-agenda-show-inherited-tags needs to do the right thing by default.

What to do? If you don't need inherited tags in
todo/search/timeline/agenda, just use this setting:

Disable parsing for some drawer properties

Text properties are used to prepare buffers for effort estimates,
appointments, and subtree-local categories in the agenda. If you
don't use some of these agenda features, you can turn them off.
Since using such drawer properties require a special scan before each
new agenda command, this can lead to a speedup. For example